replace tube on JMP-1 and question about volume

[dakisof]

Hi,

I've got both combo JTM-1 and JMP-1. Both are very good. But the JTM-1 is louder (volume) than the JMP-1 at the same volume set-up. Is that normal ? Is it due to the amp conception ?

However, I would like to know if I can swith, in the JMP-1, the two ECC81 with two ECC82 without any problem or bias set-up ? In this case, the volume would be louder or lower ?
And, to replace ECC "power" tubes in JTM-1 or JMP-1, do I need to bias (or check bias) ? And do I need to match ECC81 in the JMP-1 ?

And last thing, I've got a microphonic phenomenon on the JMP-1, but only on the E low string, 7th fret harmonic.
Do you think it's from one tube or the amp ?

Thanks a lot for answering.

My sincerest condolecences on the loss of Jim Marshall from France.

[surfnorthwest]

Welcome to the Forum dakisof

The first three Marshall 1 watt amps are cathode biased so they will bias themselves, no need to do anything. Yes you can change the two ECC81 for two ECC82 without any problem.

The amp will actually function with the following output configurations albeit with varying degrees of distortion and tonal character; -

A pair of ECC81s (12AT7) - stock production format
A single ECC81
A pair of ECC82s (12AU7)
A single ECC82

Read this link

http://marshallvintagemodern.com/viewto ... 60&t=10221

[dakisof]

Thanks a lot for your answer.

What do you think about differnece in volume between JTM-1 and JMP-1 ?
What do you think about the noise, like feedback, on low E string, 7th fret, even with guitar volume and amp volume set low ? Is it valve problem, amp problem or no problem ?

[pleximaster]

I have both amps as well, two JMPs and I feel that the JMPs are louder than JTM..
Try other tubes, or it is just amp differences

Plexi

[surfnorthwest]

Thanks a lot for your answer.

What do you think about differnece in volume between JTM-1 and JMP-1 ?
What do you think about the noise, like feedback, on low E string, 7th fret, even with guitar volume and amp volume set low ? Is it valve problem, amp problem or no problem ?

Do you have the head or the combo? If you have the combo you need to give the speaker time to break in. Tubes today are going to be mixed quality, nothing wrong with trying other tubes.

I cannot answer as to what the issue may be with the noise.

[SteveD]

Welcome to the forum dakisof.

There's no point comparing volumes between the two amps on any setting other than full as the sensitivity of the signal path may differ between the two for any other volume pot settings.

The JTM also has more bottom end like the old JTM45s and the JMP has a tighter bottom end but more upper mid agression like the 70s Super Leads. They have different characters and it depends on the tone you are dialling in which one is perceived as louder but I can assure you they both put out 1W.

Both amps also use different speakers to compliment their sound.

The noise you hear could be a rogue valve.

ECC82s in the JMP will not make it louder but they will make it cleaner though not dramatically (see my chart I posted in Surf's link). You do not need to 'match' the power valves in the JMP.

The JTM, JMP and JCM are cathode biased. But the DSL and JVM will be fixed bias.

[dakisof]

Hi Steve D,

Very thank you for your answer.

When you say that the noise could come from a "rogue" valve, do you think it could be another thing that make this noise ?

And when you say "rogue", do you mean is a faulty valve that it's better to replace or just that is a capricious valve and I can keep playing it for a while ?

[surfnorthwest]

Replace it, that way you know for sure.

[SteveD]

Yes there could be other causes of the noise but changing a valve is easily done first.

A rogue valve is a bad valve and it would be better to replace it.

[wntbtw]

Welcome to the forum dakisof.

There's no point comparing volumes between the two amps on any setting other than full as the sensitivity of the signal path may differ between the two for any other volume pot settings.

The JTM also has more bottom end like the old JTM45s and the JMP has a tighter bottom end but more upper mid agression like the 70s Super Leads. They have different characters and it depends on the tone you are dialling in which one is perceived as louder but I can assure you they both put out 1W.

Both amps also use different speakers to compliment their sound.

The noise you hear could be a rogue valve.

ECC82s in the JMP will not make it louder but they will make it cleaner though not dramatically (see my chart I posted in Surf's link). You do not need to 'match' the power valves in the JMP.

The JTM, JMP and JCM are cathode biased. But the DSL and JVM will be fixed bias.

Thank you SteveD for this post. I have the JMP1H on order and should have it soon and looking really forward to it! I am concerned about the reviews and videos about both the JMP1 and the JTM1 having alot of gain so I may 'look into' getting the ECC82 power tubes to lessen the gain--but maybe I will like the amp as it is [stock]. Can the JMP1 run w/ just 1 power tube or does it need 2? BTW this is my 2nd post [the 1st one was in the 'OBE for Jim Marshall' thread]--can I also get a 'welcome' from you? LOL! Thanks again!

[SteveD]

Ha Ha, you certainly can get a welcome from me wntbtw.

Yes you can run the power amp of the JMP with just one power valve as it is parallel single ended. If you are not into too much distortion I would recommend a single ECC82 (12AU7). Also, turning the volume down on those amps will lessen the gain without losing significant volume.

[MarshallInTown]

...
Yes you can run the power amp of the JMP with just one power valve as it is parallel single ended. If you are not into too much distortion I would recommend a single ECC82 (12AU7). Also, turning the volume down on those amps will lessen the gain without losing significant volume.

so according to your chart removing an ECC81 leaving just one wil give me even more distortion ?? Im no tech need to understand this thing better

This info applies to the JMP only!

VALVE COMPLIMENT POWER OUTPUT Total Harmonic Distortion
ECC81 (12AT7) x 2 1W 11.0%
ECC81(12AT7) x 1 1W 35.0%
ECC82 (12AU7) x 2 1W 5.5%
ECC82 (12AU7) x 1 1W 7.4%

No configuration of ECC83 (12AX7) will give the full power of 1 watt and they don't give a satisfactory option anyway IMO but for completeness here are the figures.

ECC83 (12AX7) x 2 0.6W 40.0%
ECC83 (12AX7) x 1 0.28W 40.0%

I have both JTM1C and JMP1H and Im using both with the JTM1C speaker (hooked up another cable to the 2 unused connections on the speaker) and to my ears the JMP1 is a bit louder, more compressed but louder, it also has a great attack very plexi style, both are great amps but with the additional Gain Boost the JMP1 is giving me lots o'fun although the JTM1 has a better tone... truth is I cant decide which one is better

[SteveD]

It's a bit unfair judging the JMPs tone through a speaker that was chosen for the JTM. The JMP uses a different speaker in its combo.

[MarshallInTown]

It's a bit unfair judging the JMPs tone through a speaker that was chosen for the JTM. The JMP uses a different speaker in its combo.

today I hooked up both to my 1936 2x12 loaded with Celestion/Marshall G12C 25watters and I didnt hear much difference in volume maybe JMP1 has more attack, I love how it reacts to the pick... through the 2x12 you can hear better tone and differences between the 2 amps, both are very fun to play